allow srctree header commands to be wrapped in triple string quotes to stop IDEs from complaining about them

fix "exclude_failures" mode in cythonize() by also ignoring stripped C files from compilation failures

fix code generated for __dealloc__() and __releasebuffer__() to call WriteUnraisable() on exceptions instead of building a useless traceback

make the NULL-instead-of-None change in tp_clear() optional and off by default - breaks too much code for now

Assignmment based type inference

no_gc_clear implementation (disable tp_clear) + doc and tests

Per Stefan's comment these are unneeded and actually the tests still pass.